The present invention relates to an installation for transmission of electric power via a high-voltage ac voltage line between two switchgear units located at a large distance from each other.
The invention is not limited to a certain number of phases of the ac voltage line, but this line could, for example, be a single-phase line just as well as a three-phase line.
The term “high-voltage” in this context means a system voltage exceeding at least 10 kV, and typically such a voltage is between 50 kV and 500 kV.
The definition “switchgear units located at a large distance from each other” is to be interpreted as a distance that is at least so large that the use of extruded cables with an inner electric conductor, an insulating layer of solid material surrounding this conductor, and an outer screen layer located at ground potential has been considered to be an impossible alternative for the formation of the above-mentioned ac voltage line because of the high capacitive currents that would be generated in such long cables and make the current handling capacity of the line unacceptably low. In practice, this normally means distances exceeding at least 25 km. In such installations, non-insulated overhead lines have primarily been used in order to form the ac voltage line mentioned. However, such overhead lines have a relatively great disturbing influence on nature and living beings, where they are drawn. The alternative has been to use cables with an inner conductor surrounded by a thick insulating sheath formed from oil-impregnated paper, but such cables are so expensive that they do not constitute a realistic alternative to the overhead lines. An extruded cable as mentioned above may be provided at a cost that, in combination with the advantages it exhibits with respect to environmental impact, would make it competitive with respect to the overhead lines, but it has not been possible to use it in installations of this kind because of the problems mentioned above. The conclusion has been that, to use such a cable for an ac voltage line as mentioned above, it would have been necessary to place a plurality of substations with switchgear units between the above-mentioned two switchgear units located at a large distance from each other, so that, in actual fact, the distance would not become large but instead small between two adjacently located switchgear units. This would have become unreasonably expensive and has therefore not been a conceivable solution.